1. Field of the Invention
The present invention generally relates to a method and system for cleaning a deposited metal, and more particularly to a method and system for pre-cleaning and post-cleaning a deposited nickel.
2. Description of the Related Art
In ceramic packaging technology, it has been found that refractory metals, such as tungsten and molybdenum, are not directly brazable or solderable due to their surface wettability. Generally, the external terminal refractory metal features, such as, pads and vias, must first be plated with a metal which is brazable and solderable and which can also be made to bond with molybdenum and tungsten. Usually, nickel is used as the plating metal. The common processes used for plating external features with nickel are electroplating and electroless plating in a wet bath (a so-called "wet process").
However, besides these two common "wet" plating processes (e.g., electroplating and electroless plating), a relatively new process has been developed which plates electrolessly a nickel or nickel alloy film via a dry chemical vapor deposition (CVD) process. Nickel or nickel alloy deposited in such a manner is referred to as "Dry Process Nickel" (DPN). Variations of this method are disclosed in U.S. Pat. No. 4,664,942 and in U.S. patent application Ser. No. 08/668,295, by Reddy et al., filed on Jun. 21, 1996 and assigned to International Business Machines Corporation, and the disclosure of both is incorporated herein by reference. Generally, the DPN process is a unique operation with specific attributes that are different than standard wet plating processes.
Most applications of the DPN plating involve ceramic substrates for packaging semiconductors. Traditionally, external refractory metal features of ceramic substrates have been plated with nickel using low temperature wet plating baths (e.g., with electroless nickel boron or nickel phosphorus, or electroplated nickel).
In either case, (e.g., a high temperature dry process or a low temperature wet process), the substrate surface must be cleaned appropriately and processed both before and after the respective plating step. Key contaminates which may affect the substrate surface in any plating process include extraneous metal, glass or other nonmetallic ceramic particles, such as, alumina, metal oxide, and other contaminates, such as, organic contaminates or carbonaceous residue. Each of these is addressed briefly below.
Extraneous metal is a difficult problem in substrate manufacture.
Specifically, refractory metal particles may reside on the surface of a substrate where they are not supposed to be. These particles originate from various sources including contamination in screening and lamination, and also from debris in the sintering kiln. It is very difficult, if not impossible, to completely eliminate these contaminate refractory metal particles in the manufacturing process. Therefore, these particles must be removed in subsequent processing. Generally, the particles are about 1-2 microns in size, and they are usually embedded in the surface pores of the ceramic substrate by the time the substrate has completed sintering.
Glass is usually part of the ceramic insulator, and is added intentionally to the green sheet that forms the product. Glass provides for viscous flow sintering of the ware at temperatures well below what would be required if the glass were not present. Additionally, the glass serves several other important purposes.
For example, during sintering, the glass will flow and infiltrate the via or pad. Specifically, the interface which secures the I/O pads is the glass, with "glass fingers" being formed by the glass flowing during the sintering operation. In this step, the I/O pads become securely bonded to the fired substrate. This is done basically by the glass in the pad reflowing and making contact with the glass in the substrate to form "finger links", to secure the electrical conductive pad to the ceramic substrate. However, the glass may completely (and erroneously) cover external metal features. The presence of glass on the conductive terminal features constitutes a defect, and must be removed before the plating of the terminal feature is initiated.
Similarly to glass, ceramic particles, such as alumina particles, are usually present in the metallic paste purposely, and some of these particles become exposed at the surface of the terminal metallic feature in a fired substrate. Additionally, ceramic particles may be scattered as debris over the surface of a part originating from various sources. These particles are often referred to as "fused ceramic", because they may become fused to the surface of a substrate, either metal or ceramic area, during sintering of the product.
Metal oxide is oxidation that will normally occur to a metal surface forming a thin oxide film or skin when the metal surface is exposed to an oxidizing environment. For example, even air at room temperature can be a source of terminal feature oxidation. Removal (or at least minimization) of the oxide can be a significant contributor to a plating operation's success.
Carbon debris may originate from handling or from any of a plurality of sources. Post-sintering, all organic debris should be reduced to carbonaceous residue. However, recontamination of a substrate with organic debris is an ongoing concern. Oftentimes, organic contamination will result in surface stains that disqualify the part. Additionally, carbon residue on terminal features will severely retard wettability and substrate quality, and consequently affect reliability. Organic contamination must be removed and/or avoided.
For the traditional nickel plating of electroless nickel boron or nickel phosphorous, or electroplated nickel, the extraneous refractory metal particles are removed from the substrate surface prior to plating by a complex set of chemical etches. Usually, these chemical etches include potassium ferricyanide, boiling KOH, HCL solutions, and the like. The ferricyanide dissolve the extraneous particle enough so that it is subsequently removed. The other chemical treatments prepare the terminal features for plating. If the extraneous refractory metal particles are not removed from the substrate surface prior to electroless plating, then they will plate during the plating step with nickel. For the electroless wet nickel plating, these particles will aggressively bridge to each other the plated metal creating shorts between terminal features.
For the electroplated part, the extraneous metal particles will not plate because they are electrically isolated. However, the substrate will be left with exposed refractory metal particles on its surface. This condition is unacceptable to part quality and to subsequent processing steps. Therefore, etching the refractory metal particles off the substrate surface prior to electroplating is a reasonable option.
Current pre-cleaning and post-cleaning for the DP nickel plating process also require the same complex chemical etching described above to remove extraneous refractory metal particles.
Additionally, another problem arising for post-cleaning substrates plated with DP nickel is that the plating process may detach and float nonmetallic particles associated with the metallic feature on the substrate surface, such as, for example, alumina particles, such that the nonmetallic particles will remain at the top of the feature even after it is fully plated. These particles having been floated up normally are covered or coated when using traditional plating processes such as electroless nickel boron or nickel phosphorous or electroplated nickel, and therefore are covered by the nickel plating and are of no additional concern. However, for the DP nickel plating process, these particles are problematic, and, like other nonmetallic contaminates, will degrade the wetting properties of the plated nickel film. Hence, these particles must be removed.
Currently, for DP nickel plated parts, these undesirable particles are removed, post-DPN plating, such as, by hand scrubbing each part with a brush. While hand-scrubbing is most effective in removing the undesired particles and leaving the soft, fully annealed nickel plated film undisturbed, this is a labor-intensive process and very costly. Recently, an attempt was made to replace this hand scrubbing process with a jet of water or spray. However, the particles are adhered too strongly to the substrate surface and will not be satisfactory removed by spray alone.
Thus, pre-cleaning and post-cleaning methods are performed for the DPN process. However, such a pre-cleaning requires a complex chemical etching in potassium ferricyanide, boiling KOH, HCL solutions and the like in attempting to remove glass and other debris from the surface of a metal feature to be plated, including contaminates such as metal oxide film.
Additionally, post-cleaning requires hand scrubbing with a brush. In the post-cleaning operation, two concerns are present. First, alumina particles that float to the surface of the nickel film as it is being deposited, must be removed. Secondly, extraneous nickel coated refractory metal particles that are distributed and loosely adhered to the ceramic surface where they are considered to be a defect, must be removed. The conventional method attempts to resolve these two concerns by hand scrubbing.
Thus, the conventional pre-cleaning method for DPN uses chemical etching and processing requiring expensive and environmentally-unfriendly chemicals. Further, the hand scrubbing is cumbersome, tedious, and prevents high yield. Hence, the pre-cleaning and post-cleaning steps require very labor-intensive processes and/or complex procedures and materials, thereby increasing costs and reducing yield.